The invention relates to improvements in hydrokinetic torque converters and to improvements in lockup clutches or bypass clutches for use in torque converters. More particularly, the invention relates to improvements in torque converters of the type wherein a rotary housing is provided with a chamber for a pump, a turbine, a stator and a lockup clutch having an axially movable annular piston which divides the chamber into a first compartment and a second compartment. The chamber is filled with a suitable fluid (such as oil), and the piston of the lockup clutch carries a first friction surface which can be moved into torque transmitting contact with a second friction surface when the lockup clutch is engaged. Still more particularly, the invention relates to improvements in hydrokinetic torque converters and lockup clutches wherein the first compartment is disposed between the piston and a component which carries the second friction surface, and wherein the piston and/or the aforementioned component is provided with one or more passages to establish a path for the flow of fluid from the second compartment substantially radially inwardly toward the rotational axis of the housing.
European Pat. No. 0 078 651 discloses a torque converter having a lockup clutch which includes an annular piston. That side of the piston which faces away from the friction surfaces is provided with channels serving to establish paths for the flow of fluid between a first compartment which is bounded bound by a radial wall of the housing and the piston, and the second compartment which confines the pump and the turbine of the torque converter. The direction of fluid flow is from the second compartment into the first compartment so that the fluid can cool a viscous coupling which transmits torque between the piston and the hub of the turbine.
U.S. Pat. No. 4,969,543 (granted Nov. 13, 1990 to Macdonald for "Slipping Bypass Clutch Construction for a Hydrokinetic Torque Converter") discloses a lockup clutch having an annular piston provided with a first friction surface movable against a second friction surface provided on a radially extending wall of the housing. The piston or the friction lining on the wall of the housing is provided with channels which permit a fluid to flow from the second compartment into the first compartment within the housing even when the lockup clutch is engaged. The channels are provided at the same radial distance from the rotational axis of the housing as the friction surfaces, the first compartment is disposed between the piston and the wall of the housing, and the second compartment accommodates at least the turbine of the torque converter. The patentee desires to prevent excessive thermal stressing of certain parts of the torque converter such as could develop during continuous slipping of the friction surfaces during operation of the torque converter. More specifically, the patentee desires to prevent excessive thermal stressing of parts in the region of the two friction surfaces.
Published Japanese patent application No. 58-30532 also discloses a lockup clutch or bypass clutch which is intended for use in a hydrokinetic torque converter and is provided with channels in the region of its friction surfaces.
The aforementioned patent to Macdonald is but one of numerous publications which propose the utilization of a lockup clutch whose friction surfaces slide relative to each other in the disengaged as well as in the engaged condition of the clutch. If the torque converter is installed in the power train of a motor vehicle, the slippage of the friction surfaces forming part of the lockup clutch can be short-lasting (e.g., during shifting into a different gear) or such slippage can be maintained practically within the entire operating range of the torque converter. The extent and the duration of slippage can depend upon the design of the prime mover which drives the housing of the torque converter and/or upon the selected gear ratio and/or upon one or more variable parameters of the prime mover. The lockup clutch dissipates energy in the form of heat during slippage of its friction surfaces relative to each other, and the quantity of dissipated energy can be quite pronounced (e.g., in the range of several kilowatts) during certain stages of operation of the torque converter. Such a situation can develop, for example, when a vehicle pulling a trailer is driven along a mountain road, i.e., the torque converter is apt to dissipate large amounts of energy for an extended period of time. Moreover, when the slip clutch is engaged, the amount of dissipated energy can be greatly increased, at least for a short interval of time, i.e., the torque converter and its lockup clutch are apt to be heated well above a permissible maximum temperature.
The purpose of the establishment of one or more paths for the flow of a fluid coolant is to eliminate the aforediscussed drawbacks of heretofore known torque converters and their lockup clutches. A drawback of heretofore known proposals to cool the lockup clutch of a torque converter is that the maximum torque which the lockup clutch can transmit is insufficient, and this is attributable to certain dynamic or kinetic conditions which develop in the fluid flow. The ability of conventional lockup clutches to transmit torque decreases in response to an increasing RPM of the housing of the torque converter as well as in response to increasing rate of fluid flow. This means that, if only the lockup clutch of a heretofore known torque converter is to transmit torque when the RPM of the housing rises to a preselected value, it is necessary to increase the system pressure accordingly. This, in turn, renders it necessary to employ stronger parts, such as a stronger and bulkier piston as well as a higher-capacity pump. Furthermore, the rate of fluid flow per unit of time is then increased again which results in additional losses.
The aforementioned reduction of the ability of the lockup clutches in conventional torque converters to transmit torque is attributable, among other causes, to the development of forces generated as a result of certain dynamic conditions acting upon the radially inwardly flowing fluid in a sense to increase the fluid pressure. Such forces generate a component acting in the direction of the rotational axis of the housing of the torque converter so that the piston is urged to move in a sense to disengage the lockup clutch.
A further drawback of heretofore known undertakings to cool the torque converter in the region of the lockup clutch is that the flow of cooling fluid is overly dependent upon the temperature and/or viscosity of the fluid (such as oil) and/or the difference between fluid pressures at opposite sides of the piston. This means that, if a torque converter and its lockup clutch are constructed and assembled in a manner as proposed, for example, in the aforementioned patent to air Macdonald, the resistance to the flow of fluid in the channels between the two fluid-containing compartments must be selected to be satisfactory even under critical circumstances, i.e., the rate of flow of fluid whose temperature has risen to a maximum possible or permissible value is less than the rate at which the system pressure in the torque converter would drop or collapse to an unacceptably low value. In the patented torque converter of Macdonald, the rate of fluid flow in the channels between the two compartments at opposite sides of the piston of the lockup clutch is directly dependent upon the difference between the fluid pressures in the two compartments. Such pressure differential is the variable parameter which controls the transmission of torque by the lockup clutch and, therefore, it cannot be resorted to for the selection of the desired volumetric flow of the fluid. Thus, and in order to maintain the losses in the torque converter above a minimum acceptable value, the rate of fluid flow must be low even when the difference between the fluid pressures in the two compartments rises to a maximum value, i.e., when the converter is called upon to transmit a maximal torque. This may ensure a satisfactory rate of the flow of fluid coolant when the converter is called upon to transmit maximum torque but is unsatisfactory during transmission of lesser torque because the difference between the fluid pressures in the two compartments of the torque converter is too low.